VHDR (Very High Data Rate) enabled smart cards are more and more positioned as the next generation of smart cards. New communication protocols between smart cards and smart card readers require a data rate of 1.7 Mb/s up to 13.56 Mb/s while the conventional carrier frequency of 13.56 MHz is still being used. This implies a high increase of Inter Symbol Interferences (ISI). ISI's corrupt the received waveform by interference which depends on the past transmitted symbols. As a result the present symbol is corrupted by the adjacent symbols. Equalization attempts to compensate ISI. On top of ISI, other non-idealities affect the received signal, such as DC offset, gain misadjustment and timing offsets. DC offset occurs when the mean value of the waveform is non-zero, i.e. the waveform is not centered in the available ADC range. Gain misadjustment manifests as an unknown variation of the amplitude of the waveform. Sample timing offset is the phase error between the symbol transitions and the time when the continuous waveform is sampled by the ADC.
In any receiver such non-idealities must be taken into account in order to make a correct detection. Therefore a known sequence (referred to as a preamble) is transmitted before the payload (i.e. the actual data) in order to train the receiver to cope with these non-idealities.
As a consequence, the next generation smart cards will have more integrated digital signal processing (DSP) receivers compared to today's smart cards. Furthermore, the next generation smart cards should be able to communicate with different smart card reader types, for example base stations which are connected to a power supply net and mobile devices which are NFC-enabled.
The power requirements of reader units are completely different depending on the application domain, i.e. the power requirements of a base station are very different from the power requirements of a mobile device. For example, the base station typically provides a clear signal which increases the power consumption at the transmitting side. In other words, in order to generate a clear signal the base station consumes more power. The required power is available because the base station is connected to the power supply net. If the signal is clear, the digital signal processing which takes place at the receiving side, i.e. by the DSP receiver, is relatively simple, and the preamble which is used to synchronize and calibrate the receiver is relatively short.
However, in case of a reader unit comprised in an NFC-enabled mobile device, the available power is limited and therefore the transmitted signal is less clear. In this case the digital signal processing which takes place at the receiving side is relatively complex and the required preamble is relatively long.
The DSP receiver needs to know in advance what kind of preamble it will receive. This results in different preambles in the standards of each type of reader unit. A DSP receiver is typically configured to cope only with a certain kind of preamble and, as a consequence, with a certain type of reader unit. Therefore, the DSP receiver is not very flexible, and the smart card on which it is integrated is limited in its use.